New ECM=Bad gas mileage

I recently took my 1998 Jeep Cherokee 4.0L/AT in for a new catalytic and tires. While at the shop the ECM mysteriously blew up. They had to jump the battery and I suspect they crossed the cables.

Anyway, I found a used ECM from a salvage yard with matching numbers. The only difference were the last letters. The original ECM was 537AD and the replacement is 537AH which is supposed to be an upgrade. I plugged in the replacement and it started right up. However, my gas mileage went from an average of 19 to 13! There were no trouble codes at all.

I made a few calls and everyone recommended having the ECM flashed and reprogrammed so I have. It was done recently so I havent been able to get a MPG yet.

So my question is, If the MPG is still horrible with the ECM being reprogrammed what should I check to improve the MPG?